Many of the modern electrical devices require power at voltages different from the nominal 110V or 220V supplied by utility companies to homes, offices and factories. Transformers or voltage regulators contained within the electrical devices usually provide the necessary voltage conversions. Voltage regulators also prevent surges or spikes in line voltage during start-up when an electrical device is switched on. The surges or spikes of voltage typically cause damage or failure of electrical or electronic circuits within the device unless a voltage regulator is included to control the spikes and surges. Thus, voltage regulators are important components of electrical circuits, particularly in regard to integrated circuits that are widely used in many electrical devices.
A prior art example of an on-chip voltage regulator circuit includes an operational amplifier referenced to a reference voltage (about 1.8V) that regulates the current supplying a transistor. A bandgap generator typically generates a stable voltage reference for the operational amplifier. An internal node Vdd is regulated to a midlevel voltage by the regulator circuit while an external Vdd voltage is supplied to the pin of the chip. When the current and, as a result, the voltage at the internal Vdd changes, the operational amplifier regulates a gate voltage of the transistor to supply the required current while keeping Vdd at a reference voltage.
During normal operation, since the difference between any two terminal voltages of the transistor would not exceed the reference voltage, there would not be any reliability problems. However, during startup the device capacitive members are not fully charged and the gate or the source of the transistor will approach the supply limit. This will result in a voltage corresponding to the supply limit being imposed across the gate oxide layer of the transistor, which exceeds the breakdown limit of the gate oxide and damages the transistor.